Estimating diameter increment by DBH class with horizontal point sampling data

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The estimation of diameter increment by diameter classes using methods suggested for ‘per-tree’ estimates in horizontal point sampling is discussed. The estimator usually recommended for this situation can be viewed as a ratio estimate, where the numerator is an estimate of the sum of the per-tree characteristics per hectare, and the denominator is an estimate of the number of trees per hectare having the desired characteristic. The ratio estimate is compared to the estimation of diameter increment in a DBH class by a simple arithmetic average of trees sampled. Simulation studies in a young loblolly pine plantation are used to make the comparisons. A maximum bias of about 7% occurred from using a simple arithmetic average of individual tree diameter increments. The ratio-estimate bias was always smaller than the arithmetic-average bias and was usually negligible. Variance estimators for the estimates of diameter increment were also evaluated.